Fracture toughness and strength of Al2O3-Y3Al5O12 and Al2O3-Y3Al5O12-ZrO2 directionally solidified eutectic oxides up to 1900 K

Ceramic rods of binary (Al2O3-Y3Al5O12) and ternary (Al2O3-Y3Al5O12-ZrO2) eutectic ceramic oxides were grown in air and nitrogen using the laser-heated floating zone method. Both materials presented a fine and homogeneous microstructure, free from defects, with an average interlamellar spacing of 1.1 and 0.7 micron for the binary and ternary eutectics, respectively. The strength and the toughness of the rods were measured from ambient temperature up to 1900 K by three-point bending. For the fracture tests, a sharp notch was introduced in the rods using a femto second-pulsed laser. Samples grown in nitrogen had higher strength than those grown in air. The mechanical properties of the Al2O3-YAG binary eutectic did not change with temperature up to 1500-1600 K and plastic deformation above this temperature led to a slight reduction in strength and an increase in toughness. In the case of the ternary eutectic, the toughening effect of the thermal residual stresses disappeared at high temperature and the toughness decreased by a factor of two at 1473 K. The behaviour of the ternary eutectic above this temperature followed the trends of the binary one although the changes in strength and toughness were much larger because of the smaller domain size (which favoured diffusion-assisted plastic flow) and the lower eutectic temperature. 30 refs.